The present invention relates to an acoustic microscope making it possible to analyse an object in depth and having aspherical lenses.
This microscope makes it possible to observe without damage and in depth structures of materials and in particular structures of integrated microelectronics circuits and biological cells. In the case of microelectronics, without destruction, the acoustic microscope makes it possible to test electronic components, produced e.g. on the same silicon pellet, at all stages of their production. Thus, it makes it possible to locate faults in these components, in planes parallel to the surface of the pellet or wafer, both on the surface and within said components. Furthermore, in view of the non-destructive nature of the acoustic microscope, it is possible to observe without staining a tissue sample and to, in this way, provide a rapid diagnosis with regards to the state of the tissue.
The acoustic microscope can also be used in micrometallurgy for accurately observing the relative orientation of the crystals and determining their limits without any prior polishing or chemical etching, as well as for analysing faults (microcracks, fractures) within these crystals.
The operating principle of acoustic microscopes is largely based on that of optical microscopes. This principle is in particular described in an article in Applied Physics Letters, vol. 24, no. 4, 15.2.1974 by R. A. Lemons and C. F. Quate.
Known acoustic microscopes comprise, inter alia, a focussing lens and a cylindrical objective lens, each machined at one of the ends of an acoustic propagation medium shaped like a bar and generally made from sapphire. The two bars are located in the extension of one another. Moreover, the foci of the two lenses coincide. A drop of liquid, such as water, makes it possible to link the two lenses.
A piezoelectric transducer is joined to the other end of the bars. One of the transducers makes it possible to produce ultrasonic waves within the drop of liquid, in which is immersed the object to be analysed and the other transducer makes it possible to detect the ultrasonic waves transmitted by the object. A mechanical device makes it possible to move the object in the focal plane of the two lenses, said focal plane corresponding to the observation plane of the object.
The aforementioned microscopes operate in transmission. However, microscopes operating in reflection are also known. The latter comprise a single lens either acting as a focussing lens, or as an objective lens, as well as an ultrasonic transducer acting either as a transmitter, or as a receiver.
These different microscopes make it possible to give images of objects with a definition better than 1 micron and a resolution comparable to that of the best optical microscopes.
Although these microscopes permit an excellent surface analysis of objects, it is only with difficulty that it is possible to carry out an in depth observation of the same objects therewith, which considerably limits the use thereof.
This problem encountered with depth analysis is mainly linked with the use of spherical lenses. Thus, in the bar or bars and the liquid drop there are two ultrasonic wave types which are propagated, namely longitudinal waves and transverse waves. These two types of waves, which are propagated at different speeds, the speed of longitudinal waves exceeding that of transverse waves, give rise, as a result of the spherical shape of the lenses, to two clearly different paraxial foci, one corresponding to the longitudinal waves and the other to the transverse waves. The existence of these two foci is very prejudicial to the microscopic in depth analysis of an object.
Apart from the shape of the lenses, the depth analysis of objects is difficult to perform in view of the nature of the liquid used, generally water, which has an acoustic impedence 10 to 60 times lower than that of the solid materials forming the objects to be analysed, which leads to an ultrasonic energy loss of approximately 80 to 95% making in depth observation very difficult.